Nitride semiconductors GaN, AlN, and InN and mixed crystals thereof have a wide band gap and have been used in high-power electronic devices and short-wavelength light-emitting devices. Regarding high-power devices, various technologies related to field-effect transistors (FETs), in particular, high-electron-mobility transistors (HEMTs) have been developed.
HEMTs manufactured using such nitride semiconductors have been used in high-power high-efficiency amplifiers and high-power switching devices.
HEMTs manufactured using such nitride semiconductors have an aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructure on a substrate and include a GaN layer as an electron transit layer.
The substrate may be made of sapphire, silicon carbide (SiC), gallium nitride (GaN), or silicon (Si).
GaN has a band gap of 3.4 eV, which is higher than the band gap of Si (1.1 eV) or GaAs (1.4 eV), and has a high breakdown voltage. GaN has a high saturated electron velocity and allows high-voltage and high-power operation. Thus, GaN may be used in high-voltage devices for high-efficiency switching devices and electric vehicles. In order to reduce a leakage current in transistors, a device having an insulated gate structure that includes an insulating film under a gate electrode is disclosed.    [Patent document] Japanese Laid-open Patent Publication No. 2002-359256    [Patent document] Japanese Laid-open Patent Publication No. 2010-199481
It is desirable that power switching devices be capable of normally-off operation, during which no electric current flows through a semiconductor device unless a gate voltage is applied. Normally-off operation involves a shift of the gate threshold voltage in the positive direction. Thus, structures including a p-GaN cap layer or a gate recess have been investigated.
However, a structure including a p-GaN cap layer has a problem that crystals are difficult to grow. A structure including a gate recess has a problem that it is easily damaged by etching and that it is difficult to control the depth of the gate recess.
In an insulated gate structure that has an insulating film under a gate electrode to reduce gate leakage current, the insulating film between a semiconductor layer and the gate electrode blocks an electron channel and shifts the gate threshold voltage in a negative direction. It is therefore difficult to perform normally-off operation.
The formation of a p-type NiO film on a semiconductor layer as an insulating film under a gate electrode has been studied to realize normally-off operation and reduced gate leakage current. However, NiO has a narrower band gap than SiO2 or Al2O3 and insufficiently reduces gate leakage current.